1. Field of the Invention
This invention relates to long-range, infrared, solid-state LADARs (Laser Detection and Ranging). More particularly, this invention relates to differential polarization LADARs that are as safe as possible to the human eye.
2. Description of the Background Art
Polarization can be used to improve a LADAR's ability to detect objects. Specifically, if a polarized beam is transmitted, the return beam polarization is unchanged by refection from a specular target. However, most targets are diffuse (non-specular) and therefore scatter polarized light into random polarizations. The return light can be split into vertical and horizontal polarization by a polarization beam splitter, and by use of dual detectors the ratio of random to specular return energy can be detected.
The targets of interest for most LADARs are often vehicles, which are typically painted metal objects. The reflectance of smooth metal is polarization preserving. Metal surfaces with a thin layer of paint will result in a partially polarization preserving reflectance. The variation of polarization characteristics allows classification of objects detected by the LADAR. By measuring the return energy with a vertical and a horizontal polarization detector, the relative strength of each can be determined. U.S. Pat. No. 4,333,008 entitled "Polarization Coded Doublet Laser Detection System" discloses such techniques.
This invention, as described below in detail, involves modifying the differential polarization technique described above to employ circular polarization. More particularly, the unique aspect of circular polarization light (which can be either right or left hand circularly polarized) is that reflection from a specular target reverses the sense of the circular polarization. Thus, upon reflection from a specular target, a right hand circularly polarized light beam is reversed in direction to a left hand circularly polarized light beam.
As is well known in the art, circularly polarized light is produced by the use of a quarter-wave plate. More specifically, the quarter-wave plate transforms linearly polarized light to, in essence, two bundles of light propagating in the same direction, linearly polarized and orthogonal, and shifted in phase by a quarter wavelength or 90 degrees. The superposition of the two bundles of light which are linear orthogonal polarizations of equal magnitude, and which have a 90 degree phase difference between them, produces circularly polarized light.
As is also well known in the art, when the circularly polarized light is reflected from a specular target (and therefore oppositely reversed in direction of rotation as noted above) and is propagated back through the quarter-wave plate, the quarter wave plate transforms the oppositely circularly polarized light to linearly polarized light which is orthogonal to the originally transmitted linearly polarized light. For example, vertical linearly polarized transmitted light, if reflected from a specular surface, will return as horizontal linearly polarized light. As disclosed in U.S. Pat. No. 4,844,593, this property of quarter wave plates is commonly used in optical feedback isolators of laser interferometers. This property is also employed to isolate high stability oscillators from amplifiers in multiple laser systems and in computer glare reducing screens..
In another example of a prior art application, U.S. Pat. No. 4,025,194 discloses a common aperture laser transmitter/receiver. However, this system disadvantageously employs a resonator laser beam which operates at a wavelength that is hazardous to the human eye. Moreover, this system does not employ the use of a beam expander to decrease the energy density of the laser beam. The potential for damage to the human eye is therefore significantly increased. It also does not simultaneously measure the return energy in vertical and horizontal changes to detect metal objects.
The high accuracy semiconductor laser doppler velocimeter disclosed in U.S. Pat. No. 4,919,532 advantageously employs a InGaAsP laser that produces a 1.54 micrometer wavelength light beam, which is substantially eye-safe. However, experiments have shown that a InGaAsP diode laser is undesirable because of inadequate power output for long range sensing.
In addition to the disadvantages associated with the sensors described above, it is noted that with a photodiode detector, preamplifier noise sets the limit on receiver sensitivity for a small field of view sensor. Specifically, as described below in detail, by use of an avalanche photodiode, the high sensitivity, needed for long range operation, can be achieved. The avalanche photodiode amplifies the signal before the preamp noise can degrade the signal-to-noise.
Therefore, it is an object of this invention to provide an apparatus which overcomes the aforementioned inadequacies of the prior art LADARs and provides an improvement which is a significant contribution to the advancement of the LADAR art.
Another object of this invention is to employ a diode pumped, Q-switched, ND:YLF or ND:YAG laser in an imaging LADAR to achieve a compact, lightweight LADAR sensor capable of long range operation that is as eye safe as possible.
Another object of this invention is to employ an Indium Gallium Arsenide Avalanche Photo-Diode (InGaAs APD) detector in an imaging LADAR to optimize sensitivity and minimize speckle noise.
Another object of this invention is to specifically employ a 1.32 microns operating wavelength in an imaging LADAR to minimize eye hazards.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.